1. Blown head gasket: A blown head gasket is one of the most common causes of engine oil leaking into the antifreeze reservoir. The head gasket is responsible for creating a seal between the engine block and the cylinder head. If it fails, coolant and engine oil can mix, resulting in a milky brown or foamy substance in the antifreeze reservoir.
2. Cracked engine block: A cracked engine block can also allow engine oil to leak into the antifreeze reservoir. This is a more severe issue as it requires a major engine repair or even engine replacement.
3. Faulty oil cooler: Some vehicles have an oil cooler that circulates engine oil to help regulate engine temperature. If the oil cooler fails or develops a leak, it can allow engine oil to mix with the antifreeze.
4. Defective intake manifold gasket: A defective intake manifold gasket can also lead to oil contamination in the antifreeze reservoir. The intake manifold is responsible for distributing air and fuel to the engine's cylinders. If the gasket between the intake manifold and the engine block fails, engine oil can leak into the coolant system.
5. Porous cylinder head: In rare cases, a porous cylinder head can allow engine oil to seep into the antifreeze reservoir. This issue is more common in older vehicles or those that have sustained severe engine damage.
If you notice any signs of oil in your antifreeze reservoir, such as a milky brown or foamy appearance, it's important to have your vehicle inspected by a qualified mechanic as soon as possible to identify and address the underlying issue.
